To better illustrate the nature of the invention, take for instance the case of a condensing boiler as a heat source provider (HSP). It is common to find all variety of brands and models operating at steady-state-efficiency levels from 70-80% for non-condensing to 82-98% for condensing. Steady-state Efficiency—refers to a measuring parameter for boiler maximum efficiency capability assessed under a controlled steady test and carried out by recognizable standard certification bureau. In the test, parameters such as air-intake temperature and volume, air/gas mixture, water/brine temperature/flow entering/leavening the boiler, system heat demand, and some others, are all fixed during boiler firing to obtain a better judgment of its efficiency at artificial steady state conditions. Test Standards for Gas-Fired Boilers. CGA P.2-1991 (R1999)/ENERGY START Canada, and the U.S. Department of Energy's/Title 10/Code of Federal Regulations for the Energy Conservation Program for Consumer Products, make indications that during the steady state testing of a condensing boiler water outlet temperature shall be at 180° F./82° C. and inlet temperature shall be at 80° F./26.7° C. at all times.
Drifting away from the stationary conditions dictated by the test, it arrives at the real world, a different place. A world loaded with always changing conditions where lab subsets are not so frequently encountered during the operating life span of the boiler. To complicate matters, there appears the need for adding buffer capacity in order to eliminate problems associated with excessive cycling, poor temperature control, and erratic system operation. The HVAC industry learned a long time ago that it was by adding a buffer tank to the boiler-system that they resolved all these problems. However, one issue remains unsolved. That is, the loss of the boiler high efficiency during continuous operation due to the water mixing inside the tank. But with no solution on hand, they were forced to look the other way.
In today's commercial buffers (See FIG. 2), boiler water-return at temperature tb and secondary system water-return at temperature ts easily get mixed in the buffer because of the lack of mechanical medium capable of isolating the encountering of the two flows inside the tank (See also FIG. 4). This mixed water at temperature equal tmix when going to the boiler produces the same effect on efficiency behavior as the one depicted in FIG. 3. There, and independent study (by Jim Cooke) shows how condensing and non-condensing boilers thermal efficiency gets influenced by water return temperature during steady-state conditions. Cooke's study also shows thermal efficiency behavior for a condensing boiler at three different firing rates (33/67/100%).
FIG. 4 shows some water/brine supply/return hydraulic connections for some brand name buffer tanks and their prevailing flow pattern when all intakes/outlets are in used. Water/brine motion inside the buffer not only gets affected by physical characteristics of the system such as pumps flow, buffer diameter and height, inlet/outlet configuration, among other variables, but also by changing set of dynamic conditions regulated by DCS (Distributed Control System). Flow patterns in the buffer are chaotic and unpredictable with limited opportunities for creating stratification conditions. For this to occur pumped flow coming from HSP/boiler and/or secondary system need to be slowed down to such extent that entering speed must be close to laminar flow. Only such minimal disturbance in the body of water inside the tank will have no major mixing effect in the natural convection phenomenon associated with stratification. From a design stand point this may lead to uneconomical alternatives such as having a much bigger diameter for piping inlet/outlet connections, otherwise designed with acceptable velocity of 2.1±0.9 m/s (7±3 ft/s) for normal liquid service applications, with maximum velocity of 2.1 m/s (7 ft/s) at piping discharge points. Perhaps even requiring a buffer tank with oversize uneconomical dimensions in diameter and/or height. This, without mentioning the time factor to allow the stratification process to evolve and settled in a constant demand HVAC system.
The more realistic assumption is that any flow leaving the buffer will do so at a temperature tmix.
From FIG. 2 and FIG. 4 it may be concluded that:tmix=(tb+ts)/2                tb Water/brine temperature at boiler outlet. Considered equal to t1 (See FIG. 1) when no heat losses occur in pipe connection between boiler and buffer        ts Water/brine temperature at system return. Considered equal to t2 (See FIG. 1) when no heat losses occur in pipe connection between system and buffer        tmix Water/brine temperature from the mixture of warm and hot water if there is no separation disk (as it happens in existing commercial buffers). Water temperature going to the boiler        t1 Water temperature from hot section of the buffer to the secondary system        t2 Water temperature from Secondary System to warm section of the buffer        t1-t2 Delta temperature. Q=W×Cp×(t2−t1)        Q Secondary system heat demand. Q=W×Cp×(t2−t1)        
Using data results from chart on FIG. 3 and applying the same analogy to evaluate water return/supply configuration on boiler efficiency for the typical commercial buffer connections on FIG. 4; It may be proven that when water gets mixed in the buffer and returned to the boiler at mixed temperature tmix, it will produce the same effect on the thermal efficiency of the boiler. As flow pattern and temperature of the mix evolve over time, the rising temperature of the water/brine will increasingly hamper its ability to quickly regain thermal energy when recirculating through the boiler, resulting in longer less efficient runs with increasingly unnecessary consumption of energy resources (See FIG. 6). This in turn will force chimney gases to escape the boiler without fully rendering their caloric load.
When dealing with condensing boilers it is crucial to realize that continuous 80° F./26.7° C. water-return and below is the determinant factor in achieving continuous outstanding higher efficiencies (See chart on FIG. 3); and that, boilers serving a buffer/system in which mixed water return temperature does not fall below 80° F./26.7° C. will never meet the necessary temperature requirements for achieving such continuous performance. Ignoring this fact, when justifying a boiler selection, will result in having a boiler that cost 50% more than necessary (comparing to condensing boiler) and achieves, from time to time, just above condensing boiler performance.
Currently buffer technology has not corrected the problems created with usual configurations such as the one on FIG. 4 (and the like); and as a result, its usage just exacerbate the sub-utilization of condensing boilers in boiler/buffer/systems that ONLY occasionally allow condensation to occur.